Core: Request - add Accept: 'application/json/' for json dataType
Core: Request - add Accept: 'application/json/' for json dataType

Fixes #3117

framework7io/Framework7,nolimits4web/Framework7,iamxiaoma/Framework7,iamxiaoma/Framework7,nolimits4web/Framework7,iamxiaoma/Framework7,framework7io/Framework7,nolimits4web/Framework7,iamxiaoma/Framework7